Coordinating file synchronization between a sync engine and another application that supports document collaboration

ABSTRACT

A computing system includes at least one processor and memory storing instructions executable by the at least one processor, wherein the instructions, when executed, cause the computing system to instruct a synchronization engine to synchronize first and second versions of a file, the first version being stored on a first storage system and the second version being stored on a second storage system, receive a backoff indicator corresponding to the file, based on the backoff indicator, instruct the synchronization engine to backoff synchronizing changes to the file, and allow the changes to the file to be synchronized by a co-authoring application, maintain a first set of collaborative metadata indicative of content of the first version of the file, and maintain a second set of collaborative metadata indicative of content of the second version of the file.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of and claims priority of U.S.patent application Ser. No. 15/061,124, filed Mar. 4, 2016, which isbased on and claims the benefit of U.S. provisional patent applicationSer. No. 62/193,022, filed Jul. 15, 2015, the contents of which arehereby incorporated by reference in their entirety.

BACKGROUND

There are a wide variety of different types of computing systems. Somecomputing systems run applications that allow a user to author and editdocuments. Some such applications include word processing applications,slide presentation applications, spreadsheet applications, among a widevariety of others. These types of applications often allow documents tobe stored in files in a file system.

Some computing systems have portions deployed in a cloud computing (orother remote server) environment. For instance, some computing systemsstore files on a cloud computing data storage system. However, duringauthoring and editing of a document, the files may also be stored on alocal disk by the application. Similarly, some systems have applicationsthat support document collaboration, such as merge, sharing,co-authoring, and other functionality. Some systems thus have asynchronization engine (or sync engine) that is responsible fordetecting and synchronizing changes to files and folders between a localdisk and a cloud storage system.

In order to do this, a sync engine can track the state of both the filethat is stored on the disk and that stored in the cloud, and reconcilethose states when it receives information that something has changed.For instance, if a file is edited on a local disk, the sync engine maydetect that change, realize it needs to send the change to the cloud,send the change, it may wait for the cloud to respond, and then updateits local state information to indicate that the change has been made.

A file can change in both the cloud storage system and on the localdisk. If those changes are different from one another this can create aconflict. When a conflict exists, the synchronization engine is unableto upload the disk file to the cloud storage system without overwritingwhat is in the cloud storage system. Likewise, it is unable to downloadthe cloud-based file to the local disk without overwriting the changeson the local disk. When the change to the cloud system and the localdisk are the same, then no conflict exists.

One way that the conflict problem has been addressed is to “fork” thefile. The “fork” can be created by renaming the file on the disk to aunique name, and then synchronizing down the content from the cloud, tothe local disk. The renamed file is then sent up to the cloud storagesystem as a new file. Forking thus inhibits the loss of changes, but itresults in the user having two files with changes, instead.

Another approach to handling this type of conflict is to allowapplications, themselves, to merge and co-author files. In one exampleof this type of architecture, the cloud-based storage system delegatessynchronization of the documents to an application that supportsco-authoring. For instance, when a change is received for such adocument, instead of downloading it, the cloud-based storage systemrequests the authoring application to do so. As part of this process,the co-authoring application reconciles changes between the local diskand the cloud-based storage system in order to support the co-authoringfunctionality.

This latter approach means that the cloud-based storage system and theapplication must communicate with one another in a relatively complexmanner In one example, a COM (or other) connection is established overwhich a relatively rich set of messages passes that describe the actionsthat the other party needs to take, in order to synchronize changes.This results in significant complexity. The complexity arises frommaintaining the state of the file in two places. Also, because ofdifferences in how the application and the synchronization functionallyhandle files, some large discrepancies in the maintained file states canresult.

Some applications also run in an application container that is isolatedfrom other processes. This makes the complex nature of the communicationeven more difficult.

The discussion above is merely provided for general backgroundinformation and is not intended to be used as an aid in determining thescope of the claimed subject matter.

SUMMARY

A computing system includes at least one processor and memory storinginstructions executable by the at least one processor, wherein theinstructions, when executed, cause the computing system to instruct asynchronization engine to synchronize first and second versions of afile, the first version being stored on a first storage system and thesecond version being stored on a second storage system, receive abackoff indicator corresponding to the file, based on the backoffindicator, instruct the synchronization engine to backoff synchronizingchanges to the file, and allow the changes to the file to besynchronized by a co-authoring application, maintain a first set ofcollaborative metadata indicative of content of the first version of thefile, and maintain a second set of collaborative metadata indicative ofcontent of the second version of the file.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter. The claimed subject matter is not limited to implementationsthat solve any or all disadvantages noted in the background.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one example of a file synchronizationarchitecture.

FIG. 2A is a block diagram of one example of collaborative metadata.

FIG. 2B is a flow diagram illustrating one example of the operation of asynchronization engine in the architecture shown in FIG. 1.

FIGS. 3-6 are flow diagrams illustrating various examples of theoperation of the synchronization engine under a variety of differentrace conditions.

FIG. 7 is another block diagram of the architecture illustrated in FIG.1, deployed in a different cloud computing architecture.

FIGS. 8-10 illustrate various examples of mobile devices that can beused in the architectures shown in the previous figures.

FIG. 11 is a block diagram of one example of a computing environmentthat can be deployed in the architectures shown in the previous figures.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of one example of a file synchronizationarchitecture 100. Architecture 100 illustratively includes a cloud 101and a cloud-based storage system 102 that includes one or more servers104 and data store 106 that stores files 108-110. Storage system 102 caninclude other items 112 as well. As will be described below, data store106 may store a number of different versions of the same file, and itcan store multiple different files as well.

The example of architecture 100 shown in FIG. 1 also includessynchronization engine 114. It will be noted that synchronization engine114 can be deployed in cloud 101 as well, but it is shown separately forthe sake of example. Synchronization engine 114 can include one or moreprocessors or servers 116, metadata computing logic 118, backoffprocessing logic (which may also be referred to as file lock processinglogic) 120, file synchronization logic 122, and collaborative metadatastore 124. It can also include other items 125. In one example, metadatacomputing logic 118 can maintain and communicate a set of collaborativemetadata for each file that is being synchronized. That is, each filemay have its own metadata. The collaborative metadata, for each file,can include a set of local file collaborative metadata 126 and a set ofcloud file collaborative metadata 128. Metadata 126 may illustrativelycorrespond to files 130-132 that are stored on a local file system 134(or local disk). Metadata 128 can correspond to files 108-110 that arestored on data store 106 in cloud-based storage system 102.

Architecture 100 also illustratively includes co-authoring system 136and one or more remote co-authoring systems 138. System 136illustratively generates user interfaces 140 with user input mechanisms142 for interaction by user 144. User 144 illustratively interacts withuser input mechanisms 142 in order to control and manipulateco-authoring system 136. User 146 also illustratively interacts withuser input mechanisms in order to control and manipulate remoteco-authoring application system 138.

While systems 136 and 138 may be similar systems, only system 136 willbe described in detail, for the sake of example. System 136illustratively includes one or more servers or processors 148, one ormore applications 150, backoff signal generator 151, filesynchronization logic 152, user interface component 154, and it caninclude a wide variety of other components, logic or functionality 156.In one example, servers or processors 148 illustratively run one or moreapplications 150 that can be used to author and edit documents or files.For instance, the applications 150 can include a word processingapplication, a slide presentation application, a spreadsheetapplication, an electronic mail application, a document managementapplication, etc. The applications 150 can illustratively supportdocument collaboration.

Document collaboration means several authors work on a document orcollection of documents together. They could be simultaneouslyco-authoring a document, sharing a document, or reviewing aspecification as part of a structured workflow. Document co-authoringcan involve working on a document simultaneously with one or more usersand merging changes. There are different methods of documentcollaboration and co-authoring that gradually involve more structure andcontrol around the document collaboration experience. While some formsof collaboration are described herein, the way that a given documentcollaboration system works is not important for purposes of the presentdescription and the discussion of different levels of collaboration orco-authoring is provided for the sake of example only.

Semiformal co-authoring involves multiple authors editing simultaneouslyanywhere in the document. Examples include: recurring minutes,brainstorming sessions, and reference material for a notebookapplication and team-developed financial models, budgets, and assettracking lists for a spreadsheet.

Formal co-authoring involves multiple authors editing simultaneously ina controlled way by saving content when it is ready to be revealed.Examples include: business plans, newsletters, and legal briefs for wordprocessing applications and marketing and conference presentations forslide presentation applications.

Comment and review involves a primary author soliciting edits andcomments (which can be threaded discussions) by routing the document ina workflow, but controlling final document publishing. Examples includeonline Help, white papers, and specifications.

Document sets involve authors starting workflows on an entire documentset or individual items within the document set to manage common taskssuch as review and approval.

These are only some examples of document collaboration (also referred toherein as co-authoring).

Thus, multiple authors can work on the same document, and the changescan be merged using co-authoring functionality. For instance, user 146and user 144 may be working on the same document. The changes to thedocument are illustratively synchronized either by co-authoring andmerging functionality in the application itself, or by synchronizationengine 114. In addition, metadata computing logic 118 illustrativelycomputes metadata 126 and metadata 128 for the various versions of thedocument that can be stored on local file system 134 and in cloud-baseddata store 106. This collaborative metadata, as will be discussed below,will be used to maintain the various versions of the file, insynchronization with one another.

Each system 136-138 may have its own file synchronization logic (such aslogic 152). Each system 136-138 can thus allow synchronization engine114 to synchronize changes to the various files, or it can indicate tosynchronization engine 114 that it is to backoff and not to synchronizethe changes, and, instead, the systems 136 and 138 can do it themselves.A number of examples of this are described in more detail below.

Before describing the operation of architecture 100 in more detail, abrief description of some of the items in architecture 100 will first beprovided. It is first worth noting that, in the overall operation ofarchitecture 100 sync engine 114 is primarily responsible forsynchronizing all files between file system 134 and cloud-based datastore 106. However, at any point, backoff signal generator 151 inco-authoring system 136 or system 138 can indicate to sync engine 114that it should backoff (e.g., on a file-by-file basis) and stop syncingfor a time, while the particular system 136 or 138 synchronizes files onits behalf. In one example described herein, this can be done by lockinga file so sync engine 114 cannot access it. However, it will beappreciated that backoff signal generator 151 can generate the backoffsignal as a separate signal or in other ways as well and locking thefile is only one example. While this is happening, sync engine 114receives updates about the state of the file from both system 136 andservers 104 on cloud-based storage system 102, (through thecollaborative metadata) and is able to reconcile the changes and storethem in its local collaborative metadata store 124. At some point,backoff signal generator 151 in system 136 releases sync engine 114 sothat it can resume synchronizing the file. This synchronization model isthus dramatically different and simpler than prior models. The onlymessages that system 136 and sync engine 114 are to exchange include amessage from system 136 to sync engine 114 that engine 114 is to backoff(or is released from a backoff request) and communications to get andset collaborative metadata about the file in data store 124. Further,prior models were imperative, in which one component commanded theother, whereas, in the present model, system 136 and sync engine 114 aredeclarative, meaning that they both declare their state and check thestate of the other.

Metadata computing logic 118 illustratively computes the values for thecollaborative metadata and keeps them up to date. It will be noted thatother items in architecture 100 can also compute collaborative metadata,but store 124 (which stores metadata values mastered by cloud storagesystem 102) stores a single source of truth of the metadata values forall of systems 102, and 136 and perhaps 138 (which could use anotherinstance of store 124 or even a different source for metadata).

It will be noted that collaborative metadata for a file points to thestate of the file at a certain point in time and allows the co-authoringsystem 136, sync engine 114 and storage system 102 to understand,determine, or identify a timeline relationship among the states of thefile at the different endpoints in architecture 100. In one example, thecollaborative metadata includes two pieces of data. FIG. 2A illustratesone example of this, and it is reiterated that this is just an example.FIG. 2A shows that collaborative metadata 160 (which can, for instancebe local file collaborative metadata 126 or cloud file collaborativemetadata 128 shown in FIG. 1) illustratively includes a hash value 162,a waterline value 164, and it can include other items 166. Hash value162 is a hash of the content that can be merged or co-authored in afile. This hash value 162 is illustratively shared between cloud-basedstorage system 102 and system 136, as well as sync engine 114.

Sync engine 114 can use the hash value information to determine whethera change originates locally. For instance, if the hash value for adocument downloaded from cloud storage system 102 matches that receivedfrom system 136, then sync engine 114 knows the change was made bysystem 136 and not another system 138. This, however, can be provided byan explicit “originator ID” field, which specifically identifies theoriginator of a change, in which case the hash value is not needed.Thus, it should be noted that the metadata used will depend onproperties of the co-authoring application, and can vary greatly.

With respect to the present example, all three of these items (102, 136and 114) illustratively have logic that can compute the hash value andcompare it. In another example, any of the systems or engines shown inarchitecture 100 can delegate computation of the hash value to othersystems or engines as well. For instance, in one example, sync engine114 may be configured so that it does not compute the hash value butonly compares the values generated by system 102 and system 136 todetermine equality. This type of architecture is contemplated herein.

Referring again to FIG. 2A, the waterline value illustratively uniquelyidentifies the version of a document's contents. It will also be notedthat, in one example, cloud-based storage system 102 can also maintain aseparate tag which corresponds to a version number for all files that itstores. The tag may generally be bumped or incremented when metadataabout the file changes.

The collaborative metadata illustrated in FIG. 2A can be used by system136 and system 102 for a variety of different reasons. For example, itcan use the collaborative metadata to determine if a file is up-to-dateby doing a simple equality comparison with a collaborative metadatacorresponding to the file that it currently has stored.

In another example, the systems can use the collaborative metadata toreconcile changes to a file or document. System 136, for instance, canask server 104 in storage system 102 for a base version of a documentbeing edited, by providing the corresponding waterline value 164. Server104 illustratively maintains a certain number of document versions foreach document, keyed by waterline, in data store 106. Using the baseversion provided by server 104, file synchronization logic 152 in system136 can compare the file it has on disk (e.g., in local file system 134)with the current server copy that it received from server 104 (which maybe different from the base copy) and resolve any conflicts.

This shows a significant difference between architecture 100 and priorsynchronization architectures. In some prior architectures, system 136needed to maintain a cache of the base version of all files ordocuments. With the present architecture 100, this is not needed,because system 136 can obtain the base version from storage system 102(which can be stored in store 124 and mastered by system 102). Thisdramatically improves the reliability of the computing system becausethere is a single source of truth about the document in system 102, asopposed to two different systems (system 102 and system 136) attemptingto maintain two different (and often diverging) sources of truth forthat document.

Sync engine 114 provides an interface through which systems 136 and 138can get and set file properties. Sync engine 114 illustratively knowsthe properties of the file on the local disk and needs to know if theyhave changed while sync engine 114 is backing off. System 136 uses thefunctionality on the interface to get and set properties to ensure thatit knows the properties stored in sync engine 114 and can set newproperties in sync engine 114 based on changes that system 136 has made.

This interaction is different than that between system 102 and syncengine 114. System 102 can preferentially use its knowledge of theproperties over that provided by sync engine 114. This is because theproperties sent by sync engine 114 to system 102 reflect those for thefile on the local disk. In contrast, sync engine 114 receives theproperties from the file in the cloud system from system 102.

In one example, the interface can support reading and writing moreproperties than just the collaborative metadata. For instance, theinterface may be configured so that sync engine 114 can return thecollaborative metadata (e.g., the waterline and hash), a unique fileidentifier (or resource ID), the file's last known path, and a resourcelocator that can be used for creating, changing and moving documents ona server (such as using a distributed authoring and versioning protocol)or otherwise. Also, in one example, the interface is supported so thatsystems 136 and 138 can set the collaborative metadata values (e.g., thewaterline and hash values). As is described in more detail below, in onescenario where a file already exists, system 136 requests some of theproperties and occasionally sets properties for the file. The behaviorof server 116 in reading and writing these properties is to queue a workitem in order to get or set the properties to or from the data store124.

There is also a scenario in which system 136 has a new file that system102 has not heard of and thus no database entry exists for it. Forexample, if system 136 creates a new file on local file system 134 andsets its properties in sync engine 114, but server 102 has not yet heardabout the change, server 102 is able to handle this. Server 102 handlesthis by creating an unrealized file row in its data store, with theproperties it receives from system 136. Later, when system 136 uploadsthe file, it reconciles this data with what's in the unrealized row,merges this into the main data store, and drops the unrealized row.

When an unrealized file is created in this way, system 136 provides notonly the collaborative metadata to server 102 but also the resource IDand the path for the file that it is creating. This information is usedby server 102 in creating the unrealized file entry.

The unrealized file mechanism is only one way of handling this. Otherscan be used as well.

It should also be noted that, in one example the interface betweenstorage system 102 (which can include sync engine 114) and system 136can be a COM interface, marshaled by a runtime broker, but it could beothers as well. This allows system 136 to be an isolated (or sandboxed)application that runs within a container. Thus, although system 136 maynot be able to communicate directly with system 102, it can communicatewith a runtime broker and it can thus still communicate with storagesystem 102, using the COM (or other) interface.

Because sync engine 114 acts as a store for collaborative metadata, itsynchronizes this collaborative metadata from cloud-based storage system102. In general, the collaborative metadata is updated into thecollaborative metadata store 124 in a number of different ways. First,when system 136 updates it by calling the set properties applicationprogramming interface (API) mentioned above. This action is preceded bya save of the file to disk in local file system 134. It will be noted,however, that a save to the disk does not always mean that a setproperties call will follow.

Sync engine 114 also synchronizes the collaborative metadata in datastore 124 when a change notification is received from storage system 102indicating that a change has occurred. For instance, when a file isuploaded from system 136 to data store 106, system 136 also saves thefile to disk in local file system 134. In this scenario, thecollaborative metadata is computed by storage system 102, and syncengine 114 synchronizes it to data store 124. In addition, when a fileis uploaded to data store 106 either by a website or by a clientuploading the file itself, in either of these cases, server 104illustratively computes the collaborative metadata and sync engine 114synchronizes it to its data store 124. It will be noted that, syncengine 114 need not attempt to upload collaborative metadata to storagesystem 102 in the cloud. This is because server 104 automaticallycomputes the collaborative metadata when a file on data store 106 ischanged.

FIG. 2B is a flow diagram illustrating one example of the overallarchitecture of FIG. 1. Synchronization engine 114 first receives anotification of a file change. This is indicated by block 200 in FIG.2B. It is assumed that the notification has either been received fromstorage system 102 or from system 136.

File lock processing logic 120 then determines whether system 136 hasindicated that sync engine 114 is to backoff, and not synchronize thefile (because it is doing so itself). This is indicated by block 202 inFIG. 2B. In one example, there are a variety of different ways by whichbackoff signal generator 151 in system 136 can signal sync engine 114 tobackoff. For example, logic 120 may receive a signal from generator 151indicating it should backoff. In another example, logic 120 may use tworelatively simple tests to determine whether it is to backoff. The firsttest is to determine whether the file is open in a sharing mode thatlocks the file so that sync engine 114 cannot read or delete the file.This test can be applied by making a call attempting to read the file.One example of such a call is the CreateFile call as follows:

TABLE 1 HANDLE hFile = CreateFile(filePath, GENERIC_READ | DELETE,FILE_SHARE_READ, nullptr, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL,nullptr);

If this call fails, this means that system 136 has the file openaccording to the first test.

A second test is to determine whether the file is supported forco-authoring. In such a case, logic 120 first determines whether thefile is qualified, meaning that it is associated with an applicationthat supports co-authoring. It can do this be examining the extension onthe file to determine a particular application with which the file isassociated. If the file is a qualified file, then file lock processinglogic 120 evaluates the version of the application that the file isassociated with to see whether it is a version that incorporatesco-authoring functionality. This may be done, for instance, by comparingthe extension to a list of valid extensions, or by determining theactual association. In another example, it simply determines whether theversion number is high enough to correspond to a version of theapplication that supports co-authoring. If so, then the file passes thesecond test.

If either tests fail after having previously passed, then file lockprocessing logic 120 determines that sync engine 114 is no longersupposed to backoff, and it resumes synchronizing the file. However, ifboth tests are passed, then system 136 is signaling to sync engine 114that it is to backoff, because file synchronization logic 152 in system136 is, itself, synchronizing the file.

It should be noted that these are only two example tests that can beused. Instead, for instance, file lock processing logic 120 can simplyapply the first test and determine that it can't open the file. It canthen assume that the file is co-authorable and that the application issynchronizing the file. This is contemplated herein as well.

Referring again to the flow diagram of FIG. 2B, if sync engine 114determines that system 136 is asking it to backoff, this means that theapplication 150 currently being run by system 136 will synchronize thechanges, itself. This is indicated by block 204. However, if not, thenfile synchronization logic 122 in sync engine 114 performs thesynchronization of the file changes. This is indicated by block 206.

In either case, the collaborative metadata is still updated in datastore 124. This is indicated by block 208 in FIG. 2B. In one example,the collaborative metadata is computed by file synchronization logic 152in system 136, and system 136 sets the collaborative metadata propertiesin data store 124 by calling the appropriate interface, discussed above.This is indicated by block 210. In another example, metadata computinglogic 118 in sync engine 114 computes the collaborative metadata andstores it in data store 124. This is indicated by block 212. Asdiscussed above, the collaborative metadata can be computed as hashvalue 162 which represents the content that can be merged or co-authoredin the file being synchronized. It can also be computed to includewaterline 164 which uniquely identifies the version of a document's orfile's contents. The collaborative metadata can include other items 214,as well.

Once the collaborative metadata is computed, it is synchronized to thecollaborative metadata store 124. This is indicated by block 216.

Given the different ways that collaborative metadata in data store 124is updated, a number of different race conditions can exist. These raceconditions are normally caused by the fact that system 136 can write thefile to both the cloud data store 106 and to the disk in local filesystem 134. Depending on the order in which this occurs, and the orderin which sync engine 114 sees the changes, different permutations ofraces can occur. FIGS. 3-6 are flow diagrams illustrating the operationof architecture 100 in handling a number of these different types ofrace conditions. These are examples only.

In a first race condition addressed by the flow diagram illustrated inFIG. 3, sync engine 114 is first notified that system 136 has updatedthe file on disk in local file system 134 and then receives notificationthat the file has also changed in cloud-based data store 106 (becausesystem 136 has uploaded the changes). In this case, when sync filesynchronization logic 122 in sync engine 114 receives the notificationof changes from cloud data store 106, metadata computing logic 118compares the corresponding metadata to the saved metadata that itreceived from system 136. It sees that they match so it can simplyupdate the database entry for the file to indicate that the file is insync. Therefore, file synchronization logic 122 in sync engine 114 firstdetects local updates to the disk in local file system 134. This isindicated by block 220. This can happen, for instance, when system 136saves the file to local file system 134 and indicates this to syncengine 114 and sets the collaborative metadata properties. Filesynchronization logic 122 then detects updates to the corresponding filein cloud data store 106. This is indicated by block 222.

Metadata computing logic 118 then compares the collaborative metadatafor the local and cloud versions of the file. Recall that applicationsystem 136 obtains the collaborative metadata for the file from system102 when it uploads the file and saves it to local file system 134. Ituses that collaborative metadata to call some properties on sync engine114. and provides the metadata to sync engine 114. Server 104 alsocomputes the collaborative metadata for a file when it is uploaded todata store 106 and provides that to sync engine 114. Metadata computinglogic 118 compares them and finds that they match because they both camefrom the same source (i.e., server 104). This is indicated by block 224.Metadata computing logic 118 then updates the collaborative metadata andsynchronizes it to data store 124 to indicate that the files are insync. This is indicated by blocks 226 and 227.

FIG. 4 is a flow diagram illustrating processing in architecture 100 fora second race condition. In the second race condition addressed by FIG.4, sync engine 114 first sees the updates to a file in cloud data store106 and then the updates on the local disk in local file system 134. Inthis case, metadata computing logic 118 compares the last known state ofthe collaborative metadata for the local file on file system 134 withwhat it downloaded from the cloud based on the changes to the fileuploaded to data store 106. It will see that they do not match (becauseit has not yet seen the changes to the local file system 134) and thusmarks the file to be in a deferred download state. In that case, thedownload will only be started if the file is no longer open by system136. The deferred download state will be cleared if and when the updatesfrom system 136 are seen. Often, system 136 will update thecollaborative metadata in sync engine 114 so that the collaborativemetadata will match and the file will not be downloaded. However, if,for instance, system 136 crashes before it can send the collaborativemetadata update, or, for some other reason, does not send thecollaborative metadata update, sync engine 114 will recover the file bydownloading the file (since it was deferred and will be periodicallyre-tried) from cloud data store 106 and replacing the file on disk inthe local file system 134, thus ensuring a correct and stable state.

Thus, in FIG. 4, file synchronization logic 122 in sync engine 114 firstdetects updates to the cloud file and corresponding updates to thecollaborative metadata for the cloud file. This is indicated by block230 in FIG. 4. It then compares the last known state of thecollaborative metadata for the local file with the collaborativemetadata for the cloud file and finds that they do not match. This isindicated by block 232. File synchronization logic 122 thus marks thefile for deferred download, as indicated by block 234. If the file hasbeen closed in system 136, then the file is no longer marked fordeferred download. This is indicated by block 236. However, if not, filesynchronization logic 122 can still look for updates to the local file,in which case the collaborative metadata for the local file will also beupdated in data store 124. This is indicated by block 238. In eithercase, the deferred download state for the file is cleared as indicatedby block 240.

If system 136 has updated the collaborative metadata for the local fileso that it matches that for the cloud file, then processing iscompleted, and the file does not need to be downloaded the file to thelocal disk. This is indicated by block 242. However, if, for somereason, system 136 did not update the collaborative metadata for thelocal file, then file synchronization logic 122 downloads the file fromthe cloud data store 106 and replaces the local file on the disk inlocal file system 134, with the downloaded file and updates thecollaborative metadata for the local file. This is indicated by blocks244 and 245. It will also be noted that, when system 136 did not updatethe collaborative metadata and it was downloaded by sync engine 114, thedefined downloaded state may be cleared after the download is complete,instead of earlier as indicated by block 240.

Thus, even if system 136 crashed before it updates the collaborativemetadata in sync engine 114, the correct version of the file will besynchronized from data store 106 to the local file system 134. Both setsof collaborative metadata will be updated.

FIG. 5 is a flow diagram illustrating the operation of architecture 100in handling yet another race condition. In this race condition, system136 makes two edits. The first is uploaded to storage system 102, butnot saved to disk in local file system 134. The second edit is uploadedto data store 106 and saved on local file system 134, and thecorresponding collaborative metadata is computed and written to datastore 124 in sync engine 114. Sync engine 114 sees the property update(based on the second edit) first, and then sees two notifications, onefor each upload to data store 106. Of course, the first notificationwill be stale, as the second notification supersedes it. Thus, in FIG.5, file synchronization logic 122 first detects and records thecollaborative metadata update from system 136, based upon it saving thefile to local file system 134 (recall that this is from the secondedit). This is indicated by block 250. It then receives the firstnotification that the cloud file has been updated on data store 106(recall that this is from the first edit). This is indicated by block252. It then compares the collaborative metadata for the local and cloudfiles and finds that they do not match. This is indicated by block 254.It thus marks the file for deferred download, because it is open insystem 136. This is indicated by block 256. If this download is everattempted, it will fail because file synchronization logic 122 does nothave a version tag that matches the version of the file on data store106 (because it has not yet seen the second notification fromcloud-based data storage system 102). Then, it sees the secondnotification from storage system 102. This is indicated by block 258. Itthen compares the collaborative metadata for the cloud file and thelocal file, and this time it finds that they match. This is indicated byblock 260. Therefore, it cancels the deferred download state for thefile and the file is written into the data store as being up-to-date.This is indicated by block 262.

FIG. 6 is a flow diagram illustrating the operation of architecture 100in addressing yet another race condition. In the race conditionaddressed by FIG. 6, system 136 creates a file and uploads it tocloud-based data store 106 but does not write it to the local disk inlocal file system 134. Sync engine 114 then receives the notificationfor the change in cloud data store 106. System 136 then makes an edit tothe file and uploads that to data store 106, but crashes before it canwrite the file to disk in local file system 134. Sync engine 114 thusreceives the notification for the edit to cloud data store 106 andproperly synchronizes the file to local file system 134.

In this case, file synchronization logic 122 receives the firstnotification of the new file uploaded to cloud store 106. This isindicated by block 270 in FIG. 6. It then compares the collaborativemetadata and finds that no match exists, and it marks the file fordeferred download. This is indicated by block 272.

Logic 122 then receives the second notification representative of theedit that was uploaded to the cloud store 106. This is indicated byblock 274. Metadata computing logic 118 compares the collaborativemetadata and finds that still, no match exists. The file marking fordeferred download is maintained. This is indicated by block 276. Whensystem 136 crashes, file lock processing logic 120 determines that thesystem has crashed (or that it is no longer open) and the deferreddownload operation is performed. This is indicated by blocks 278 and280. File synchronization logic 122 thus downloads the current versionof the file from data store 106 to the local file system 134. It updatesthe collaborative metadata in data store 124 to reflect this. This isindicated by block 282.

It will be noted that FIGS. 3-6 show processing in accordance with avariety of different race conditions. Similar permutations to thesetypes of conditions can be created when files are created instead ofedited. This is slightly different, because it causes sync engine 114 tocreate an unrealized file entry in the data store if it hasn't heardabout the change. Otherwise, the flows are similar to those describedabove.

It can thus be seen that even if system 136 does not write the file tothe local disk or send updated collaborative metadata properties to syncengine 114, sync engine 114 properly recovers and downloads the fileitself. The user thus always ends up with a correct and stable versionof the file. Similarly, even if system 136 fails to upload the file tothe cloud data store 106, but manages to write it to disk in the localfile system 134 (regardless of whether it sends the updatedcollaborative metadata properties), sync engine 114 properly recoversand uploads the file itself. Again, the user ends up in a correct andstable state.

Further, there is only one source of truth about the file, and it ismaintained in collaborative metadata store 124. This increasesreliability.

Similarly, sync engine 114 serializes updates from cloud-based storagesystem 102 and from system 136, and handles them consistently. In onecase, sync engine 114 hears about the new properties from system 136,and then from the cloud. However, if this is not the scenario, syncengine 114 handles the updates correctly anyway.

In addition, file change conflicts can occur. If they do, the behaviorof architecture 100 depends upon whether sync engine 114 or system 136,itself, is synchronizing the file. If system 136 is synchronizing thefile, then the conflict resolution behavior or functionality in theapplication 150 running on system 136 will take effect. This can includemerging the documents, surfacing them for user conflict resolution, etc.If sync engine 114 is synchronizing the document, it can fork the filein the case of a conflict, or choose another resolution. For instance,it can rename the local file on local file system 134 to a new, uniquename, and download a server copy from data store 106, in its place. Allof these are contemplated herein.

The present discussion has mentioned processors and servers. In oneembodiment, the processors and servers include computer processors withassociated memory and timing circuitry, not separately shown. They arefunctional parts of the systems or devices to which they belong and areactivated by, and facilitate the functionality of the other componentsor items in those systems.

Also, a number of user interface displays have been discussed. They cantake a wide variety of different forms and can have a wide variety ofdifferent user actuatable input mechanisms disposed thereon. Forinstance, the user actuatable input mechanisms can be text boxes, checkboxes, icons, links, drop-down menus, search boxes, etc. They can alsobe actuated in a wide variety of different ways. For instance, they canbe actuated using a point and click device (such as a track ball ormouse). They can be actuated using hardware buttons, switches, ajoystick or keyboard, thumb switches or thumb pads, etc. They can alsobe actuated using a virtual keyboard or other virtual actuators. Inaddition, where the screen on which they are displayed is a touchsensitive screen, they can be actuated using touch gestures. Also, wherethe device that displays them has speech recognition components, theycan be actuated using speech commands.

A number of data stores have also been discussed. It will be noted theycan each be broken into multiple data stores. All can be local to thesystems accessing them, all can be remote, or some can be local whileothers are remote. All of these configurations are contemplated herein.

Also, the figures show a number of blocks with functionality ascribed toeach block. It will be noted that fewer blocks can be used so thefunctionality is performed by fewer components. Also, more blocks can beused with the functionality distributed among more components.

It will be noted that the above discussion has described a variety ofdifferent systems, components and/or logic. It will be appreciated thatsuch systems, components and/or logic can be comprised of hardware items(such as processors and associated memory, or other processingcomponents, some of which are described below) that perform thefunctions associated with those systems, components and/or logic. Inaddition, the systems, components and/or logic can be comprised ofsoftware that is loaded into a memory and is subsequently executed by aprocessor or server, or other computing component, as described below.The systems, components and/or logic can also be comprised of differentcombinations of hardware, software, firmware, etc., some examples ofwhich are described below. These are only some examples of differentstructures that can be used to form the systems, components and/or logicdescribed above. Other structures can be used as well.

FIG. 7 is a block diagram of architecture 100, shown in FIG. 1, exceptthat its elements are disposed in a different cloud computingarchitecture 500. Cloud computing provides computation, software, dataaccess, and storage services that do not require end-user knowledge ofthe physical location or configuration of the system that delivers theservices. In various embodiments, cloud computing delivers the servicesover a wide area network, such as the internet, using appropriateprotocols. For instance, cloud computing providers deliver applicationsover a wide area network and they can be accessed through a web browseror any other computing component. Software or components of architecture100 as well as the corresponding data, can be stored on servers at aremote location. The computing resources in a cloud computingenvironment can be consolidated at a remote data center location or theycan be dispersed. Cloud computing infrastructures can deliver servicesthrough shared data centers, even though they appear as a single pointof access for the user. Thus, the components and functions describedherein can be provided from a service provider at a remote locationusing a cloud computing architecture. Alternatively, they can beprovided from a conventional server, or they can be installed on clientdevices directly, or in other ways.

The description is intended to include both public cloud computing andprivate cloud computing. Cloud computing (both public and private)provides substantially seamless pooling of resources, as well as areduced need to manage and configure underlying hardware infrastructure.

A public cloud is managed by a vendor and typically supports multipleconsumers using the same infrastructure. Also, a public cloud, asopposed to a private cloud, can free up the end users from managing thehardware. A private cloud may be managed by the organization itself andthe infrastructure is typically not shared with other organizations. Theorganization still maintains the hardware to some extent, such asinstallations and repairs, etc.

In the example shown in FIG. 7, some items are similar to those shown inFIG. 1 and they are similarly numbered. FIG. 7 specifically shows thatstorage system 106 and sync engine 114 are located in cloud 502 (whichcan be public, private, or a combination where portions are public whileothers are private). Therefore, user 144 uses a user device 504 thatincludes component 506 to access those systems through cloud 502.

FIG. 7 also depicts another example of a cloud architecture. FIG. 7shows that it is also contemplated that some elements of architecture100 are disposed in cloud 502 while others are not. By way of example,data store 106 can be disposed outside of cloud 502, and accessedthrough cloud 502. In another example, sync engine 114 is also outsideof cloud 502. Regardless of where they are located, they can be accesseddirectly by device 504, through a network (either a wide area network ora local area network), they can be hosted at a remote site by a service,or they can be provided as a service through a cloud or accessed by aconnection service that resides in the cloud. All of these architecturesare contemplated herein.

It will also be noted that architecture 100, or portions of it, can bedisposed on a wide variety of different devices. Some of those devicesinclude servers, desktop computers, laptop computers, tablet computers,or other mobile devices, such as palm top computers, cell phones, smartphones, multimedia players, personal digital assistants, etc.

FIG. 8 is a simplified block diagram of one illustrative example of ahandheld or mobile computing device that can be used as a user's orclient's hand held device 16, in which the present system (or parts ofit) can be deployed, and which can be used in architecture 100. FIGS.9-10 are examples of handheld or mobile devices.

FIG. 8 provides a general block diagram of the components of a clientdevice 16 that can run components architecture 100 or that interactswith architecture 100, or both. In the device 16, a communications link13 is provided that allows the handheld device to communicate with othercomputing devices and under some embodiments provides a channel forreceiving information automatically, such as by scanning. Examples ofcommunications link 13 include an infrared port, a serial/USB port, acable network port such as an Ethernet port, and a wireless network portallowing communication though one or more communication protocolsincluding General Packet Radio Service (GPRS), LTE, HSPA, HSPA+ andother 3G and 4G radio protocols, 1Xrtt, and Short Message Service, whichare wireless services used to provide cellular access to a network, aswell as Wi-Fi protocols, and Bluetooth protocol, which provide localwireless connections to networks.

In other examples, applications or systems are received on a removableSecure Digital (SD) card that is connected to a SD card interface 15. SDcard interface 15 and communication links 13 communicate with aprocessor 17 (which can also embody processors or servers from FIG. 1)along a bus 19 that is also connected to memory 21 and input/output(I/O) components 23, as well as clock 25 and location system 27.

I/O components 23, in one embodiment, are provided to facilitate inputand output operations. I/O components 23 for various embodiments of thedevice 16 can include input components such as buttons, touch sensors,multi-touch sensors, optical or video sensors, voice sensors, touchscreens, proximity sensors, microphones, tilt sensors, and gravityswitches and output components such as a display device, a speaker, andor a printer port. Other I/O components 23 can be used as well.

Clock 25 illustratively comprises a real time clock component thatoutputs a time and date. It can also, illustratively, provide timingfunctions for processor 17.

Location system 27 illustratively includes a component that outputs acurrent geographical location of device 16. This can include, forinstance, a global positioning system (GPS) receiver, a LORAN system, adead reckoning system, a cellular triangulation system, or otherpositioning system. It can also include, for example, mapping softwareor navigation software that generates desired maps, navigation routesand other geographic functions.

Memory 21 stores operating system 29, network settings 31, applications33, application configuration settings 35, data store 37, communicationdrivers 39, and communication configuration settings 41. Memory 21 caninclude all types of tangible volatile and non-volatilecomputer-readable memory devices. It can also include computer storagemedia (described below). Memory 21 stores computer readable instructionsthat, when executed by processor 17, cause the processor to performcomputer-implemented steps or functions according to the instructions.Similarly, device 16 can have a client system 24 which can run variousbusiness applications or embody parts or all of architecture 100.Processor 17 can be activated by other components to facilitate theirfunctionality as well.

Examples of the network settings 31 include things such as proxyinformation, Internet connection information, and mappings. Applicationconfiguration settings 35 include settings that tailor the applicationfor a specific enterprise or user. Communication configuration settings41 provide parameters for communicating with other computers and includeitems such as GPRS parameters, SMS parameters, connection user names andpasswords.

Applications 33 can be applications that have previously been stored onthe device 16 or applications that are installed during use, althoughthese can be part of operating system 29, or hosted external to device16, as well.

FIG. 9 shows one example in which device 16 is a tablet computer 600. InFIG. 6, computer 600 is shown with user interface display screen 602.Screen 602 can be a touch screen (so touch gestures from a user's fingercan be used to interact with the application) or a pen-enabled interfacethat receives inputs from a pen or stylus. It can also use an on-screenvirtual keyboard. Of course, it might also be attached to a keyboard orother user input device through a suitable attachment mechanism, such asa wireless link or USB port, for instance. Computer 600 can alsoillustratively receive voice inputs as well.

Additional examples of devices 16 can be used as well. Device 16 can be,a feature phone, smart phone or mobile phone. The phone can include aset of keypads for dialing phone numbers, a display capable ofdisplaying images including application images, icons, web pages,photographs, and video, and control buttons for selecting items shown onthe display. The phone can include an antenna for receiving cellularphone signals such as General Packet Radio Service (GPRS) and 1Xrtt, andShort Message Service (SMS) signals. In some examples the phone alsoincludes a Secure Digital (SD) card slot that accepts a SD card.

FIG. 10 shows that the device can be a smart phone 71. Smart phone 71has a touch sensitive display 73 that displays icons or tiles or otheruser input mechanisms 75. Mechanisms 75 can be used by a user to runapplications, make calls, perform data transfer operations, etc. Ingeneral, smart phone 71 is built on a mobile operating system and offersmore advanced computing capability and connectivity than a featurephone.

Note that other forms of the devices 16 are possible.

FIG. 11 is one example of a computing environment in which architecture100, or parts of it, (for example) can be deployed. With reference toFIG. 11, an example system for implementing some embodiments includes ageneral-purpose computing device in the form of a computer 810.Components of computer 810 may include, but are not limited to, aprocessing unit 820 (which can comprise processors or servers from otherFIGS.), a system memory 830, and a system bus 821 that couples varioussystem components including the system memory to the processing unit820. The system bus 821 may be any of several types of bus structuresincluding a memory bus or memory controller, a peripheral bus, and alocal bus using any of a variety of bus architectures. By way ofexample, and not limitation, such architectures include IndustryStandard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus,Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA)local bus, and Peripheral Component Interconnect (PCI) bus also known asMezzanine bus. Memory and programs described with respect to FIG. 1 canbe deployed in corresponding portions of FIG. 11.

Computer 810 typically includes a variety of computer readable media.Computer readable media can be any available media that can be accessedby computer 810 and includes both volatile and nonvolatile media,removable and non-removable media. By way of example, and notlimitation, computer readable media may comprise computer storage mediaand communication media. Computer storage media is different from, anddoes not include, a modulated data signal or carrier wave. It includeshardware storage media including both volatile and nonvolatile,removable and non-removable media implemented in any method ortechnology for storage of information such as computer readableinstructions, data structures, program modules or other data. Computerstorage media includes, but is not limited to, RAM, ROM, EEPROM, flashmemory or other memory technology, CD-ROM, digital versatile disks (DVD)or other optical disk storage, magnetic cassettes, magnetic tape,magnetic disk storage or other magnetic storage devices, or any othermedium which can be used to store the desired information and which canbe accessed by computer 810. Communication media typically embodiescomputer readable instructions, data structures, program modules orother data in a transport mechanism and includes any informationdelivery media. The term “modulated data signal” means a signal that hasone or more of its characteristics set or changed in such a manner as toencode information in the signal. By way of example, and not limitation,communication media includes wired media such as a wired network ordirect-wired connection, and wireless media such as acoustic, RF,infrared and other wireless media. Combinations of any of the aboveshould also be included within the scope of computer readable media.

The system memory 830 includes computer storage media in the form ofvolatile and/or nonvolatile memory such as read only memory (ROM) 831and random access memory (RAM) 832. A basic input/output system 833(BIOS), containing the basic routines that help to transfer informationbetween elements within computer 810, such as during start-up, istypically stored in ROM 831. RAM 832 typically contains data and/orprogram modules that are immediately accessible to and/or presentlybeing operated on by processing unit 820. By way of example, and notlimitation, FIG. 11 illustrates operating system 834, applicationprograms 835, other program modules 836, and program data 837.

The computer 810 may also include other removable/non-removablevolatile/nonvolatile computer storage media. By way of example only,FIG. 11 illustrates a hard disk drive 841 that reads from or writes tonon-removable, nonvolatile magnetic media, and an optical disk drive 855that reads from or writes to a removable, nonvolatile optical disk 856such as a CD ROM or other optical media. Other removable/non-removable,volatile/nonvolatile computer storage media that can be used in theexemplary operating environment include, but are not limited to,magnetic tape cassettes, flash memory cards, digital versatile disks,digital video tape, solid state RAM, solid state ROM, and the like. Thehard disk drive 841 is typically connected to the system bus 821 througha non-removable memory interface such as interface 840, and optical diskdrive 855 are typically connected to the system bus 821 by a removablememory interface, such as interface 850.

Alternatively, or in addition, the functionality described herein can beperformed, at least in part, by one or more hardware logic components.For example, and without limitation, illustrative types of hardwarelogic components that can be used include Field-programmable Gate Arrays(FPGAs), Program-specific Integrated Circuits (ASICs), Program-specificStandard Products (ASSPs), System-on-a-chip systems (SOCs), ComplexProgrammable Logic Devices (CPLDs), etc.

The drives and their associated computer storage media discussed aboveand illustrated in FIG. 11, provide storage of computer readableinstructions, data structures, program modules and other data for thecomputer 810. In FIG. 11, for example, hard disk drive 841 isillustrated as storing operating system 844, application programs 845,other program modules 846, and program data 847. Note that thesecomponents can either be the same as or different from operating system834, application programs 835, other program modules 836, and programdata 837. Operating system 844, application programs 845, other programmodules 846, and program data 847 are given different numbers here toillustrate that, at a minimum, they are different copies.

A user may enter commands and information into the computer 810 throughinput devices such as a keyboard 862, a microphone 863, and a pointingdevice 861, such as a mouse, trackball or touch pad. Other input devices(not shown) may include a joystick, game pad, satellite dish, scanner,or the like. These and other input devices are often connected to theprocessing unit 820 through a user input interface 860 that is coupledto the system bus, but may be connected by other interface and busstructures, such as a parallel port, game port or a universal serial bus(USB). A visual display 891 or other type of display device is alsoconnected to the system bus 821 via an interface, such as a videointerface 890. In addition to the monitor, computers may also includeother peripheral output devices such as speakers 897 and printer 896,which may be connected through an output peripheral interface 895.

The computer 810 is operated in a networked environment using logicalconnections to one or more remote computers, such as a remote computer880. The remote computer 880 may be a personal computer, a hand-helddevice, a server, a router, a network PC, a peer device or other commonnetwork node, and typically includes many or all of the elementsdescribed above relative to the computer 810. The logical connectionsdepicted in FIG. 11 include a local area network (LAN) 871 and a widearea network (WAN) 873, but may also include other networks. Suchnetworking environments are commonplace in offices, enterprise-widecomputer networks, intranets and the Internet.

When used in a LAN networking environment, the computer 810 is connectedto the LAN 871 through a network interface or adapter 870. When used ina WAN networking environment, the computer 810 typically includes amodem 872 or other means for establishing communications over the WAN873, such as the Internet. The modem 872, which may be internal orexternal, may be connected to the system bus 821 via the user inputinterface 860, or other appropriate mechanism. In a networkedenvironment, program modules depicted relative to the computer 810, orportions thereof, may be stored in the remote memory storage device. Byway of example, and not limitation, FIG. 11 illustrates remoteapplication programs 885 as residing on remote computer 880. It will beappreciated that the network connections shown are exemplary and othermeans of establishing a communications link between the computers may beused.

It should also be noted that the different embodiments described hereincan be combined in different ways. That is, parts of one or moreembodiments can be combined with parts of one or more other embodiments.All of this is contemplated herein.

Example 1 is a computing system, comprising:

file synchronization logic that synchronizes changes to a file that hasa corresponding cloud file stored on a cloud based storage system and acorresponding local file stored on a local file system;

backoff processing logic that receives a backoff indicator correspondingto the file and determines, based on the backoff indicator, whether thefile synchronization logic is to synchronize the changes to the file orto backoff and allow the changes to be synchronized by a co-authoringapplication and

metadata computing logic that maintains a first set of collaborativemetadata indicative of content and a version of the cloud file and asecond set of collaborative metadata indicative of content and a versionof the local file.

Example 2 is the computing system of any or all previous exampleswherein the backoff processing logic determines whether the filesynchronization logic is to backoff by determining whether the file isopen by a co-authoring application and, if so, indicates to the filesynchronization logic that the file is locked.

Example 3 is the computing system of any or all previous exampleswherein the computing system exposes a metadata interface that can beinvoked by the co-authoring application to obtain and set thecollaborative metadata for the local file.

Example 4 is the computing system of any or all previous exampleswherein the metadata computing logic maintains the first set ofcollaborative metadata by generating a first hash value that is a hashof the content of the cloud file and a first waterline value thatuniquely identifies a version of the contents of the cloud file.

Example 5 is the computing system of any or all previous exampleswherein the file synchronization logic detects changes to the cloud fileand changes to the local file and, in response, the metadata computinglogic compares the first set of collaborative metadata to the second setof collaborative metadata and, if there is a match, determines that thecloud file and the local file are synchronized with one another.

Example 6 is the computing system of any or all previous exampleswherein when the file synchronization logic detects updates to the cloudfile, but does not detect changes to the local file, the metadatacomputing logic determines that the first and second set ofcollaborative metadata do not match and the backoff processing logic isconfigured to determine that the file is locked until either the file isclosed in the co-authoring application or the until the second set ofcollaborative metadata for the local file is updated by the co-authoringapplication.

Example 7 is the computing system of any or all previous exampleswherein the file synchronization logic is configured to, in response todetecting a change to the local file or the cloud file, and in responseto the metadata computing logic determining that the first and secondsets of collaborative metadata do not match, determine which of thecloud file and the local file changed and synchronize the cloud filewith the local file, based on which file changed.

Example 8 is the computing system of any or all previous exampleswherein the metadata computing logic maintains the first and second setsof collaborative metadata regardless of whether the backoff processinglogic determines that the file is locked.

Example 9 is a computer implemented method of synchronizing a file,wherein the file is stored on a cloud-based storage system as acloud-based file, and on a local file system as a corresponding localfile, comprising:

receiving notification that the cloud-based file or the local file haschanged;

determining whether an application, associated with the file, has thefile open;

if the application has the file open, then determining that theapplication is to synchronize the cloud-based file with the local file;

if the application does not have the file open, then synchronizing thecloud-based file and the local file, based on the changes; and

updating a first set of collaborative metadata corresponding to thecloud-based file and a second set of collaborative metadatacorresponding to the local file in a metadata store accessible by theapplication and the cloud-based storage system, the first and secondsets of collaborative metadata identifying a version and content of thecorresponding file.

Example 10 is the computer implemented method of any or all previousexamples wherein determining whether the application has the file opencomprises:

determining whether the application supports co-authoring functionality.

Example 11 is the computer implemented method of any or all previousexamples wherein determining whether the application has the file opencomprises:

determining whether the application has the file open in a co-authoringmode.

Example 12 is the computer implemented method of any or all previousexamples wherein determining whether the application has the file openin a co-authoring mode, comprises:

attempting to perform an operation with respect to the file that isprecluded when the document is open in the application in theco-authoring mode; and

determining whether the operation is successful.

Example 13 is the computer implemented method of any or all previousexamples wherein updating the first and second sets of collaborativemetadata, for each set of collaborative metadata, comprises:

updating a hash value that comprises a hash of content in thecorresponding file that can be co-authored.

Example 14 is the computer implemented method of any or all previousexamples wherein updating the first and second sets of collaborativemetadata, for each set of collaborative metadata, comprises:

updating a waterline value that uniquely identifies a version of thecontent of the corresponding file.

Example 15 is the computer implemented method of any or all previousexamples wherein updating the first and second sets of collaborativemetadata comprises:

exposing a metadata interface that can be invoked by the application toobtain and set the collaborative metadata for the local file.

Example 16 is the computer implemented method of any or all previousexamples wherein determining that the application is to synchronize thecloud-based file with the local file, comprises:

marking the file for a deferred download for synchronization after theapplication closes the file;

after the application no longer has the file open, comparing the firstand second sets of collaborative metadata;

if the first and second sets of collaborative metadata match, thendetermining that the cloud-based file and the local file are alreadysynchronized; and

if the first and second sets of collaborative metadata do not match,then synchronizing the cloud-based file and the local file.

Example 17 is the computer implemented method of any or all previousexamples wherein updating the first and second sets of collaborativemetadata comprises:

storing the first and second sets of collaborative metadata in themetadata store that comprises a single source of the first and secondsets of collaborative metadata for both the cloud-based storage systemand the application.

Example 18 is a computing system, comprising:

file synchronization logic that synchronizes changes to a file that hasa corresponding cloud file stored on a cloud based storage system and acorresponding local file stored on a local file system;

backoff processing logic that determines whether the file is open by aco-authoring application and, if so, indicates to the filesynchronization logic that the file is locked; and

metadata computing logic that maintains a first set of collaborativemetadata indicative of content and a version of the cloud file and asecond set of collaborative metadata indicative of content and a versionof the local file in a metadata store that comprises a single source forthe stored first and second sets of collaborative metadata for both theco-authoring application and the cloud based storage system.

Example 19 is the computing system of any or all previous exampleswherein once the file is unlocked, the metadata computing logic isconfigured to determine whether the first and second sets ofcollaborative metadata match one another, if so, the filesynchronization logic is configured to determine that the co-authoringapplication has already synchronized the changes.

Example 20 is the computing system of any or all previous exampleswherein the metadata computing logic is configured to maintain the firstset of collaborative metadata by generating a first hash value that is ahash of the content of the cloud file and a first waterline value thatuniquely identifies a version of the contents of the cloud file, andmaintains the second set of collaborative metadata by maintaining asecond hash value that is a hash of the content of the local file and asecond waterline value that uniquely identifies a version of thecontents of the local file.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed is:
 1. A computing system comprising: at least oneprocessor; and memory storing instructions executable by the at leastone processor, wherein the instructions, when executed, cause thecomputing system to: instruct a synchronization engine to synchronizefirst and second versions of a file, the first version being stored on afirst storage system and the second version being stored on a secondstorage system; receive a backoff indicator corresponding to the file;based on the backoff indicator, instruct the synchronization engine tobackoff synchronizing changes to the file, and allow the changes to thefile to be synchronized by a co-authoring application; and maintain afirst set of collaborative metadata indicative of content of the firstversion of the file; and maintain a second set of collaborative metadataindicative of content of the second version of the file.
 2. Thecomputing system of claim 1, wherein the instructions cause thecomputing system to: determine whether the synchronization engine is tobackoff synchronizing changes by determining that the file is open bythe co-authoring application and, in response, indicating to thesynchronization engine that the file is locked.
 3. The computing systemof claim 1 wherein the computing system exposes a metadata interfacethat is invoked by the co-authoring application to obtain and set thesecond set of collaborative metadata for the second version of the file.4. The computing system of claim 3 wherein the instructions cause thecomputing system to: maintain the first set of collaborative metadata bygenerating a first hash value that is a hash of the content of the firstversion of the file and a first waterline value that uniquely identifiesa version of the contents of the first version of the file.
 5. Thecomputing system of claim 3 wherein the instructions cause the computingsystem to: detect a first change to the first version of the file and asecond change to the second version of the file and, in response,compare the first set of collaborative metadata to the second set ofcollaborative metadata; and based on identifying a match between thefirst and second sets, determine that the first version of the file andthe second version of the file are synchronized with one another.
 6. Thecomputing system of claim 3 wherein the instructions cause the computingsystem to: determine that the first and second set of collaborativemetadata do not match; and determine that the file is locked untileither the file is closed in the co-authoring application or the untilthe second set of collaborative metadata for the second version of thefile is updated by the co-authoring application.
 7. The computing systemof claim 3 wherein the instructions cause the computing system to: inresponse to determining that the first and second sets of collaborativemetadata do not match, determine which of the first version and thesecond version have changed and synchronize the first version of thefile with the second version of the file, based on which versionchanged.
 8. The computing system of claim 3 wherein the instructionscause the computing system to: maintain the first and second sets ofcollaborative metadata regardless of whether the file is locked.
 9. Amethod performed by a computing system, the method comprising:instructing a synchronization engine to synchronize first and secondversions of a file, the first version being stored on a first storagesystem and the second version being stored on a second storage system;receiving a backoff indicator corresponding to the file; based on thebackoff indicator, instructing the synchronization engine to backoffsynchronizing changes to the file, and allow the changes to the file tobe synchronized by a co-authoring application and maintaining a firstset of collaborative metadata indicative of content of the first versionof the file; and maintaining a second set of collaborative metadataindicative of content of the second version of the file.
 10. The methodof claim 9, and further comprising: determining whether thesynchronization engine is to backoff synchronizing changes bydetermining that the file is open by the co-authoring application and,in response, indicating to the synchronization engine that the file islocked.
 11. The method of claim 10, and further comprising: maintainingthe first set of collaborative metadata by generating a first hash valuethat is a hash of the content of the first version of the file and afirst waterline value that uniquely identifies a version of the contentsof the first version of the file.
 12. The method of claim 10, andfurther comprising: detecting a first change to the first version of thefile and a second change to the second version of the file and, inresponse, comparing the first set of collaborative metadata to thesecond set of collaborative metadata and, based on identifying a match,determining that the first version of the file and the second version ofthe file are synchronized with one another.
 13. The method of claim 10,and further comprising: determining that the first and second set ofcollaborative metadata do not match; and determining that the file islocked until either the file is closed in the co-authoring applicationor the until the second set of collaborative metadata for the secondversion of the file is updated by the co-authoring application.
 14. Themethod of claim 10, and further comprising: in response to determiningthat the first and second sets of collaborative metadata do not match,determining which of the first version of the file and the secondversion of the file having changed and synchronizing the first versionof the file with the second version of the file, based on which versionchanged.
 15. The method of claim 10, and further comprising: maintainingthe first and second sets of collaborative metadata regardless ofwhether the file is locked.
 16. A computing system comprising: at leastone processor; and memory storing instructions executable by the atleast one processor, wherein the instructions, when executed, cause thecomputing system to: receive notification that at least one of the firstversion or the second version has changed; determine whether anapplication, associated with the file, has the file open; based ondetermining that the application has the file open, determine that theapplication is to synchronize the first version with the second version;based on determining that the application does not have the file open,synchronize the first version and the second version; and update a firstset of collaborative metadata corresponding to the first version and asecond set of collaborative metadata corresponding to the second versionin a metadata store accessible by the application and the first storagesystem, the first and second sets of collaborative metadata identifyingcontent of the corresponding version.
 17. The computing system of claim16, wherein the instructions cause the computing system to: determinethat the application supports co-authoring functionality and has thefile open in a co-authoring mode.
 18. The computing system of claim 18,wherein the instructions cause the computing system to determine thatthe application has the file open in a co-authoring mode by: attemptingto perform an operation with respect to the file that is precluded whenthe document is open in the application in the co-authoring mode; anddetermining whether the operation is successful.
 19. The computingsystem of claim 16 wherein the instructions cause the computing systemto: determine that the application is to synchronize the first versionwith the local file by: marking the file for a deferred download forsynchronization after the application closes the file; after theapplication no longer has the file open, comparing the first and secondsets of collaborative metadata; if the first and second sets ofcollaborative metadata match, then determining that the first versionand the second version are already synchronized; and if the first andsecond sets of collaborative metadata do not match, then synchronizingthe first version and the second version.
 20. The computing system ofclaim 16 wherein the instructions cause the computing system to: storingthe first and second sets of collaborative metadata in the metadatastore that comprises a single source of the first and second sets ofcollaborative metadata for both the first storage system and theapplication.